1. Field of the Invention
The present invention relates to a switching power supply having power factor correction.
2. Description of the Related Art
In switching power supplies, correction of a power factor requires suppression of generation of harmonics of AC input current and reduction of phase difference between AC input current and AC input voltage. As this kind of switching power supply, there has hitherto been developed one in which AC voltage is switched by a pulse width modulated (PWM) drive signal (refer to Japanese Patent Laid-Open Nos. 2001-238452 and 2001-045763, and Liu XueChao and Wang ZhiHao, “UCC28070 Implement Bridgeless Power Factor Correction Pre-Regulator Design” Texas Instruments Application Report JAJA214 http://focus.tij.co.jp/jp/lit/an/jaja214/jaja214.pdf).
In a switching power supply having power factor correction (PFC switching power supply), for example, positive half cycle and negative half cycle of AC voltage are each properly switched by use of a switching device driven by a PWN drive signal having a frequency substantially higher than AC power line frequency, whereby a desired DC voltage is provided and in addition, generation of harmonics in AC input is suppressed and the phase difference between AC input current and AC input voltage is reduced to correct the power factor. In this case, “positive half cycle of AC voltage” means half cycle in which the L pole (Live pole) side has positive voltage relative to N pole (Neutral pole) of AC voltage; and “negative half cycle of AC voltage” means half cycle in which the L pole (Live pole) side has negative voltage relative to N pole (Neutral pole) of AC voltage.
As a configuration of bridgeless power factor correction circuit, for example, six types illustrated in FIGS. 15(A) to 15(F) are known (in the circuits of FIG. 15, the capacitors are each a smoothing capacitor, and the circuit section contributing to power factor correction is constituted of coils, switching devices and rectifying devices and others).
The aforementioned PFC switching power supplies have a function of driving the switching devices of the power factor correction section based on results of determining whether it is in positive or negative half cycle of AC voltage (in this case, the circuits illustrated in FIG. 15 each correspond to a booster convertor, and the section for determination on positive or negative half cycle is not illustrated).
However, a circuit element lies between N and L poles of AC voltage and reference potential of the power factor correction section (typically, ground potential of the power factor correction section, that is, reference potential on the side of DC output); consequently, a difference of voltage lies therebetween. In determining the polarity of AC voltage, this difference of voltage is an undesirable factor. Thus, in order to remove effects of this difference of voltage, a circuit element (DC isolator), such as a transformer or photo coupler, is used to perform DC isolation between N and L poles of AC voltage and reference potential point of the power factor correction section (Japanese Patent Laid-Open Nos. 2001-238452 and 2001-045763).
However, when the element for performing DC isolation between the AC input side and the DC output side is used, the size of the circuit for determination of polarity of AC voltage is enlarged. Further, the use of a transformer, hardly downsized, causes problems of size and weight growth of the PFC switching power supply, resulting in increased cost, as the AC power line frequencies are low (for example, 50 Hz or 60 Hz). Furthermore, when different AC voltages or different AC power line frequencies are handled, it is also needed to provide plural sorts of transformers different in specifications design.
In implementing the power factor correction section having various circuit configurations, different sorts of hardware are needed for each circuit configuration, thus causing problems of increasing development/design cost and manufacturing cost and also increasing the number of components used in maintenance.
In order to address the above problems, an object of the present invention is to implement a PFC switching power supply which can determine the polarity of AC voltage and detect the voltage thereof without using a DC isolator such as a transformer or photo coupler and preferably in which hardware related to determination of polarity of AC voltage can be used in a shared manner.